1. Field of Invention
The present invention relates generally to semiconductor devices and methods for fabricating the same. More particularly, at least some embodiments are directed to flip chip semiconductor packages, and packaging processes, that incorporate under-die cavities.
2. Discussion of Related Art
Radio frequency integrated circuits (RFICs) are widely used in wireless devices, such as cellular telephones. RFICs combine transmission lines, matching networks, and discrete components, such as inductors, resistors, capacitors and transistors, on a substrate to provide a subsystem capable of transmitting and receiving high frequency signals, for example, in a range of about 0.1 to 100 Gigahertz (GHz). Packaging of RFICs is distinctly different from packaging of digital ICs due to the fact that the package is often part of the RF circuit, and because the complex RF electrical and/or magnetic fields of the RFIC can interact with any nearby insulators and conductors. To meet growing demands in the wireless industry, RFIC packaging development seeks to provide smaller, lower cost, higher performance devices that can accommodate multi-die RF modules while providing high reliability and using lead-free solder and other “green” materials. The single chip package, in which single- or multi-die RFICs are individually packaged, is a direct solution to the small size and low cost requirements of RFICs, and is currently used for most RFICs.
Micro electromechanical systems (MEMS) enable controlled conversions between micro-scale mechanical motion and specified electrical signals, for example, with specified frequencies. MEMS are becoming widely used in RFICs. Based on mechanical movements, RF MEMS can achieve excellent signal quality factors for RF band filters, including surface acoustic wave (SAW) filters, bulk acoustic wave (BAW) filters, and high frequency RF switches. SAW filters, for example, convert electrical signals into a mechanical wave that is delayed as it propagates across a piezoelectric crystal substrate before being converted back into an electrical signal. BAW filters use volume bulk movement to achieve a specific desired resonance, and in RF switches, electrical signals are used to control movement of a micro-electrode to turn the switch ON or OFF. Current MEMS technologies have evolved from semiconductor fabrication processing. However, the mechanical motion uniquely associated with MEMS demands very different packaging constructions and requirements from conventional semiconductor ICs. In particular, inside all MEMS ICs, some materials must move freely, without interference, and therefore, MEMS ICs must be “capped” to form a small vacuum or air cavity around the moving materials to protect them while permitting their movements.
One example of a package for an RF MEMS device, developed by Infineon Technologies, AG, uses a complex passivation structure to create an air cavity underneath a SAW/BAW filter die. A photolithographic polymer is used to generate a maze structure forming a cavity for each resonator. Reverse wire bonds are used to make the interconnections between the filter die and substrate. A silicon lid, e.g., a generally flat silicon wafer, with B-stage adhesive is attached on top of the maze structure to “cap” the ICs and complete the enclosed cavities. This package has been a relatively effective MEMS package as it uses standard die attach and wirebonding assembly technologies. However, it limits package size reduction, and the additional process steps of maze patterning and lid attachment add considerable complexity and cost to the package which reduces package efficiency and increases cost.